Using an external drive but its not longer showing in Finder

I have recently bought an Imation 1t.b M100 bac-up drive, I succesully moved photos and audio and docs onto it the first time I connected. Since then, the drive has not shown up in Finder, the drive is working as its light is on and whirring, but willnot show in Finder.

I do not have the original Mac discs, so cannot run Disc Utility. Is there anything I can do to show it up in Finder?

Thanks in advance.